As known, the so-called “negative” yarn-feeders comprise a stationary drum on which a motorized swivel flywheel winds a plurality of yarn loops forming a weft reserve or stock. Upon request from the textile machine, the loops are unwound from the drum, then pass through a weft-braking device which controls the tension of the yarn, and finally are fed to the machine which, with regard to the present invention, preferably consists of a circular/rectilinear knitting machine of a conventional type.
The yarn-feeders of the above type, which are well-known to the person skilled in the art, have the main aim of maintaining the amount of yarn stored on the drum substantially constant, while minimizing the tension of the yarn delivered from the drum.
The amount of yarn stored on the drum is controlled by a triad of sensors. A first sensor, typically a Hall sensor, detects the passing of magnets attached to the flywheel in order to calculate the amount of yarn wound on the drum and the winding speed; a second sensor, generally a mechanical sensor, provides a binary information indicative of the presence or absence of a minimum amount of stocked loops in the area where the sensor is arranged; a third sensor, which can be, e.g., a optical sensor, a piezoelectric sensor, and the like, provides at least one pulse per each unwound loop, and is also used for calculating the amount of yarn wound on the drum and the winding speed.
While with the so-called “positive” yarn feeders such as the one described in EP-A-950742, the tension of the yarn is directly controlled by comparing a reference tension value with a measured tension value, and then by varying the yarn-feeding speed in such a way as to minimize the difference between such values, with the negative yarn feeders the tension is controlled either by weft-braking devices such as the one described in EP-B-534 263, or by devices having a simpler construction, such as brush-type brakes or so-called “duck-type” brakes of a conventional type.
In braking devices such as the one described in EP-B-622 485, the yarn is pressed between a fixed lamina and a movable braking member, which is also shaped as a lamina and is driven by a linear motor. In braking devices such as the one described in EP-B-1 059 375, the unwinding yarn is pressed between the delivery edge of the drum and a frustoconical, hollow braking member connected to a motor. In both cases, the motor which drives the braking member is controlled by a closed-loop control unit which modulates the braking action applied upon the yarn. The control unit receives a measured tension signal from a tension sensor arranged downstream of the feeder, and compares it with a reference tension indicative of the desired tension, by a control loop having the aim of minimizing the difference between the measured tension and the reference tension.
The above-described control system is designed to compensate the slow variations of tension due, for example, to wearing of the braking means, and is set to be substantially unaffected by small, sudden variations of tension caused, e.g., by the presence of a knot or by the passing of a length of yarn having an uneven section.
However, with certain operative conditions, e.g., at the starting of the weaving process, when the knitting machine is not running, or at the threading step, when the yarn is motionless, the above control system is subject to deceiving because the tension of the yarn unwinding from the feeder is much lower than the normal operative tension, and in certain cases it may be even equal to zero. In these cases, the control loop increases the intensity of the braking action more and more up to the uppermost braking level, without ever reaching the desired tension value. Consequently, when yarn is drawn from the drum again, such a high braking value causes the yarn tension to reach a peak that can give rise to textile defects and even to the breaking of the yarn.